Inductive heating method utilizing high frequency harmonics and intermittent cooling

ABSTRACT

Heating systems and methods for inductive heating or a combination of resistive and inductive heating. A heater coil is inductively coupled to an article and a current signal is supplied to the heater coil. The heater coil generates a magnetic flux, based on the applied current signal, for inductively heating the article. Current pulses of a certain profile are used to enhance the rate, intensity and/or power of inductive heating delivered by the heating element or coil and/or to enhance the lifetime or reduce the cost of the inductive heating system.

RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.10/884,851, filed Jul. 2, 2004, and U.S. patent application Ser. No.10/612,272, filed Jul. 2, 2003, the contents of which are incorporatedherein by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to heating systems and methods which include, invarious implementations, utilizing inductive heating or a combination ofresistive and inductive heating; furthermore, the heating may belocalized (directed to particular areas), and/or the heating may becontinuous or intermittent.

BACKGROUND OF THE INVENTION

It is common practice to inductively heat a cylinder or tube of aferromagnetic (high magnetic permeability) material, such as steel, byan induction (eddy) current. The eddy current is induced in theferromagnetic material by an applied magnetic flux, and the magneticflux is generated by passage of an alternating current through one ormore heater coils disposed around the cylinder or tube. This method ofinductive heating can be adapted to various other types of materials,work pieces and loads, including fluid, semisolid or solid materials(e.g., molten steel or magnesium filled and non-filled polymers, billetsand ceramics).

The article to be heated may itself be heated by an induction current,and/or it may be in thermal communication (e.g., by conduction orradiation) with another article or substrate being inductively heated,for example, when heat inductively generated in a ferromagneticsubstrate is transferred to a semiconductor wafer. In this regard, theelectrical resistivity of the heating element or coil may be varied, forexample using a more resistive material to increase the amount ofresistive heat generated in the coil and transferred to the article (byconduction or radiation). Nichrome, a nickel chromium (NiCr) alloyhaving about sixty times the electrical resistivity of copper, has beenused for the coil to generate both a magnetic flux for inductive heatingof an article lying within the flux, and resistive heat (in the coil),which is then transferred by conduction and/or radiation to the samearticle.

Traditional inductive heating coils are made of copper and are watercooled to prevent overheating of the coil. Also, an air gap is providedbetween the water-cooled coil and the article being heated, to avoidremoval of heat from the article by the coil cooling medium. The air gapand cooling requirements increase the complexity and cost of the heatingsystem. They also reduce the strength (structural integrity) of theapparatus, which can be critical in applications where pressure isapplied, e.g., a compression mold. However, without cooling, the coil issubject to failure (melting or burn out at elevated temperatures).Traditional inductive heating systems do not utilize more highlyresistive (e.g., NiCr) coils, because the enhanced resistive heating ofthe coil would make coil cooling even more difficult, requiring stilllarger cooling channels and/or lower cooling temperatures, each of whichresults in greater energy consumption and cost. Furthermore, a resistiveload cannot be driven by a traditional inductive power supply.

There is an ongoing need for heating systems and methods which addresssome or all of these problems and/or for energy sources to power suchheating systems more efficiently and preferably at a lower cost.

SUMMARY OF THE INVENTION

Systems and methods consistent with the present invention include thefollowing implementations.

According to one implementation, a heating apparatus includes a heatercoil inductively coupled to an article and a current signal is suppliedto the heater coil. The heater coil generates a magnetic flux, based onthe current pulse signal, for inductively heating the article. Thecurrent signal is preferably a current pulse signal with high frequencyharmonics.

The high frequency harmonics may be used to vary the inductive heatingpower. The harmonics may enhance a relative proportion of inductiveheating, compared to resistive heating, of the heater coil. Thehigh-frequency harmonics may enable use of a lower fundamental (or root)frequency supply current (e.g., line frequency of 50-60 Hz). Theeffective frequency of the current pulse, based on a combination of theroot and harmonic frequency components, and their amplitudes, mayenhance the lifetime of the heater coil in particular applicationsand/or enable more rapid heating of the coil.

In one embodiment, a heater coil is inductively coupled to a load whichincludes the article. The load includes a ferromagnetic core andferromagnetic yoke, and the heater coil is in contact with, disposedbetween, and/or embedded within at least one of the core and yoke. Insome cases the core has a passage for a flowable material, such that thecore heats the flowable material. The heater coil may be positioned inthe core so that heating is concentrated in the passage.

In another implementation, an article forms at least a part of asubstantially closed loop for the magnetic flux. The article includes afirst portion in which inductive heating is more concentrated, comparedto a second portion of the article. The second portion may causediscontinuities in, or restrict the flow of, the eddy current, forexample, by having slots, air gaps or a less ferromagnetic material inthe second portion.

In another implementation, a power source is provided which supplies acurrent signal to a heater coil. The current is preferably supplied ascurrent pulses with an adjustable harmonics energy content to the heatercoil.

In one embodiment, a heater coil is positioned at least partially withinan article having a passage for a flowable material to be heated, andheat generated inductively in the article is delivered by conductionand/or convection to the flowable material in the passage. The powersource delivers current pulses which vary in amplitude and/or frequencyspectrum (frequencies of the harmonics), to the heater coil foradjusting the delivery of inductive heating to the flowable material inthe passage. The flowable material may itself be ferromagnetic such thateddy current are induced in the material (in addition to or instead ofin the article).

According to another implementation, a method is provided which includesthe steps of providing a heater coil inductively coupled to an article,and providing a current signal to the heater coil. The current signal ispreferably a current pulse signal with high frequency harmonics.

According to another implementation, the method steps include providinga heater coil in thermal communication with and inductively coupled toan article, and providing an adjustable current pulse signal to theheater coil for adjusting the ratio between inductive and resistiveheating of the article.

According to various implementations, the method steps may includesimultaneous, discontinuous, intermittent and/or alternating periods ofheating, cooling, and/or temperature control; adjusting the energycontent of the current pulse signals with respect to amplitude, pulsewidth and/or frequency spectrum; and/or providing a cooling mechanism(cooling medium or heat sink) to withdraw heat from the article beingheated. Particular structures are disclosed for accomplishing thesemethod steps. Various embodiments of such heating systems and methodsmay provide one or more benefits such as more uniform heating, reducedthermal gradients, reduced thermal stresses, reliable high temperationoperation, compact design, shorter cycle time, and reduced heat-up time.

These and other implementations will be described in the followingfigures and detailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a cross-sectional schematic view of one implementation of aheating system for providing both inductive and resistive heating,wherein a wrapped heating coil is embedded between a co-axial innerferromagnetic core and outer ferromagnetic yoke in order to provide aclosed magnetic loop (see arrows) and enhanced magnetic coupling betweenthe coil and core/yoke;

FIG. 1B is an enlarged fragmentary view of encircled section 1B of FIG.1A, showing the electrically-insulated coil disposed in a groove in thecore;

FIG. 1C is a partial, broken away side view, of a second implementationof a heating system similar to that shown in FIG. 1A but with slots inthe yoke;

FIG. 1D is a sectional view taken along line 1D-1D of FIG. 1C showinginduction (eddy) currents in the core directed oppositely to the currentin the coil, and showing discontinuities in the eddy current in the yokebecause of the slots;

FIG. 1E is a schematic view of a barrel extruder with multipletemperature zones which may incorporate the heating systems of FIGS.1A-1D;

FIG. 2 is a general schematic diagram of a power supply providingcurrent pulses with high-frequency harmonics to a heating system of thetype shown in FIGS. 1A-1D, according to one implementation of theinvention;

FIG. 3A is a circuit diagram of a power supply using thyristors toprovide the current pulses;

FIG. 3B is a circuit diagram of a power supply using gate-turn-off (GTO)thyristors to provide the current pulses;

FIG. 3C is a circuit diagram of a power supply using an integrated gatebipolar transistor (IGBT) device to provide the current pulses;

FIG. 4A is a timing diagram illustrating the current pulses generatedfrom a line frequency current supply by the thyristors of FIG. 3A;

FIG. 4B is a timing diagram illustrating the current pulses generatedfrom a line frequency current supply by the GTO thyristors of FIG. 3B,

FIG. 4C is a timing diagram illustrating the current pulses generatedfrom a line frequency current supply by the IGBT device of FIG. 3C;

FIG. 5A is a circuit diagram of a three-phase, three-pulse unipolarcommutator providing additional current pulses from additional phases ofthe line frequency supply, and FIG. 6A is the associated timing diagram;

FIG. 5B is a circuit diagram of a three-phase, six-pulse bipolarcommutator providing additional current pulses from additional phases ofthe line frequency supply, and FIG. 6B is the associated timing diagram;

FIG. 5C is a circuit diagram of a one-phase, two-pulse unipolar pulsatorproviding additional current pulses from the bridge circuit of the linefrequency supply, and FIG. 6C is the associated timing diagram;

FIG. 5D is a circuit diagram of a three-phase, six-pulse unipolarpulsator providing additional current pulses from additional phases ofthe line frequency supply, and FIG. 6D is the associated timing diagram;

FIG. 5E is a circuit diagram of a three-phase, twelve-pulse unipolarpulsator providing additional current pulses from the line frequencysupply, and FIG. 6E is the associated timing diagram;

FIG. 7 is an isometric view of a heating system used in an experimentfor comparing the heating performance of a sinusoidal line frequencycurrent versus current pulses with high-frequency harmonics, and FIG. 7Ais an enlarged cross-sectional view taken along line 7A-7A of FIG. 7;

FIG. 8 is a temperature/time graph of recorded data from the experimentconducted with the heating system of FIG. 7, showing a substantiallyhigher rate of heating with the current pulses as compared to asinusoidal current;

FIG. 9 is a schematic profile of the current pulses used in theexperiment of FIG. 7;

FIG. 10 is a cross-sectional schematic view of another implementation inwhich the heating apparatus is incorporated into a furnace, and FIG. 10Ais an enlarged fragmentary view of the encircled section 10A in FIG. 10;

FIG. 11 is a cross-sectional schematic view of another implementation,in which the heating apparatus is incorporated into a water heater orchemical reactor;

FIG. 12 is a schematic diagram of another implementation, in which theheating apparatus is incorporated as heater patches that are surfacemounted on a chemical container or reactor;

FIG. 13 is a cross-sectional schematic view of one implementation of alayered heating structure;

FIG. 14 is cross-sectional schematic view of another implementation of alayered heating structure, having an inner ferromagnetic layer ofthickness A;

FIG. 15 is cross-sectional schematic view of another implementation of alayered heating structure, with cooling passages in the innerferromagnetic layer;

FIG. 16 is cross-sectional schematic view of another implementation of alayered heating structure, with cooling passages in the heating element;

FIG. 17 is cross-sectional schematic view of another implementation of alayered heating structure, where the inner layer includes acorrosion-resistant and thermally-conductive liner;

FIG. 18 is cross-sectional schematic view of another implementation of alayered heating structure, formed by a thermal spray method, and FIG.18A is an enlarged fragmentary view of the encircled section 18A in FIG.18;

FIG. 19 is cross-sectional schematic view of another implementation of alayered heating structure, formed by a thermal spray method, and FIG.19A is an enlarged fragmentary view of the encircled section 19A in FIG.19;

FIG. 20 is an exploded parts view of an injection nozzle assembly with acoiled heater element;

FIG. 21 is a sectional view taken along line 21-21 of FIG. 22;

FIG. 22 is an end view (at the mold end) of the nozzle assembly of FIG.20;

FIG. 23 is an exploded parts view of a multi-temperature zone nozzleassembly, having upper and lower serpentine conductor patterns;

FIG. 24 is an elevational side view of the assembled nozzle of FIG. 23;

FIG. 25 is a sectional view taken along line 25-25 in FIG. 24;

FIG. 25A is an enlarged fragmentary view of the encircled section 25A inFIG. 25;

FIG. 26 is an elevational view of one half of a blow-molding apparatus;

FIG. 27 is an exploded parts view of the blow-molding apparatus of FIG.26;

FIG. 28 is a schematic fragmentary view of the heating element andadjacent layers of the apparatus of FIG. 27;

FIG. 29 is a timing diagram of a blow-molding and thermal-conditioningmethod;

FIG. 30 is a schematic perspective view of a compression mold;

FIG. 31 is an exploded parts view of the compression mold of FIG. 30;

FIG. 32 is a schematic fragmentary view of the heating element andadjacent layers of the compression mold of FIG. 30;

FIG. 33 is a schematic cross-sectional view of the assembled componentsof FIG. 31;

FIG. 34 is an enlarged fragmentary view of the encircled section 34 inFIG. 33;

FIG. 35 is an enlarged fragmentary view of the encircled section 35 inFIG. 34;

FIG. 36 is a timing diagram of a compression molding method;

FIG. 37 is a schematic perspective view of a cylindrical heater coilwrapped around a core;

FIG. 38 is a schematic cross-sectional view of the coil and core of FIG.37;

FIG. 39 is a schematic elevational view of a planar spiral coil mountedon a plate;

FIG. 40 is a cross-sectional schematic view of the coil and plate ofFIG. 39;

FIG. 41 is a schematic elevational view of a planar looped spiral coilmounted on a plate;

FIG. 42 is a cross-sectional schematic view of the coil and plate ofFIG. 41;

FIG. 43 is a schematic elevational view of a planar serpentine coilmounted on a plate;

FIG. 44 is a cross-sectional schematic view of the coil and plate ofFIG. 43;

FIG. 45 is a schematic elevational view of a planar looped serpentinecoil mounted on a plate;

FIG. 46 is a cross-sectional schematic view of the coil and plate inFIG. 45;

FIG. 47 is a graph of amplitude versus time showing a single sine wave;

FIG. 48 is a graph of the frequency spectrum of a sine wave of FIG. 47;

FIG. 49 is a graph of amplitude versus time showing a current pulsesignal with high frequency harmonics;

FIG. 50 is a graph showing the frequency spectrum of the current pulsesignal of FIG. 49;

FIG. 51 is a schematic view of a control circuit for a heating andcooling apparatus.

DETAILED DESCRIPTION

It has been determined that current pulses of a certain profile can beused in various embodiments described herein to enhance the rate,intensity and/or power of inductive heating delivered by a heatingelement or coil and/or to enhance the lifetime or reduce the cost of aninductive heating system. This may be accomplished, in selectembodiments, without requiring a corresponding increase of current inthe heater coil. It may also enable use of a lower frequency (e.g.,50-60 Hz) supply current and may be coupled with structural heating andcooling elements that enable directed (localized) heating and coolingeffects for producing tighter temperature control or a reduced cycletime.

More specifically, these current pulses have a rapidly changing currentprofile which enhances the inductive heating performance. The currentpulses are discrete narrow width pulses with steep edges (large firstderivatives), which include harmonics of a fundamental or root frequencyof the coil current. These harmonics, above the root frequency, aredescribed herein as high frequency harmonics, which preferably occurabove the border frequency of the heating coil and/or heating system.The provision of such pulses to a heater coil may be used tosignificantly increase the power inductively delivered to aferromagnetic or other inductively heated load, without requiring anincrease of the Root Mean Square (RMS) current in the coil. This may inturn decrease the energy consumption or cooling requirements of and/orincrease the lifetime of the heater coil.

One problem that may be addressed by use of these current pulses, aloneor coupled with the structural heating and cooling elements describedherein, is the maximum tolerable coil current, or limit current(I_(C-limit)) which a heater coil can withstand and still provide auseful lifetime. Thus, for a given I_(C-limit)(RMS), number of coilturns N, and coefficient of electromagnetic connection K_(C), oneproblem addressed here is how to increase the inductive heating power.

In the prior art, a solution is to increase the frequency of the powersupply, in which case powerful capacitors are provided in parallel withthe coil as a “resonant converter” to adjust (tightly control) theresonant frequency of the sinusoidal current supplied to the heatercoil. One problem with this solution is that the power supply is notadapted to work with a resistive load (resistive coil).

Furthermore, the prior art's use of inductive heating for surfaceheating requires tight control of the depth of penetration, which inturn requires tight control of the frequency. As a result, harmonics area disfavored and consequently insignificant (minimized) portion of thecurrent signal supplied to the heater coil. This is consistent with thegeneral disfavor of high frequency harmonics—e.g., when providingsinusoidal 60 Hz line current, the current providers use huge capacitorsto rid their systems of harmonics because their customers do not wantharmonics, referred to as noise, in the supplied signal interfering withtheir electrical equipment and computers, and altering the effectivefrequency.

In contrast here, current pulses are deliberately provided withharmonics above the root frequency of the coil current. These discretenarrow current pulses have steep edges (changes in amplitude) andrelatively long delays between pulses. They appear as a chopped orcompressed wave with a relatively large delay between pulses in eachcycle.

The harmonics provide an increase in the effective frequency of thecurrent pulse signal, particularly where the amplitudes of the harmonicsare kept high so that the inductive heating power is high. Viewed with aspectrum analyzer, the current pulses would include multiple currentcomponents, at each of multiple harmonic frequencies. It is understoodas used herein that current and voltage are interchangeable andequivalent.

Preferably, the harmonics are above the border frequency of the coil orheating system, and the root frequency of the current pulse signal isalso preferably above the border frequency (as the root frequency mayprovide the largest amplitude component of the current pulse signal).The amplitudes of the harmonics may be enhanced, for example, by use ofa transformer or the like. Various implementations are described belowof systems and methods for configuring the current pulse signals, aswell as select applications illustrating their use.

One benefit of this approach can be the provision of a simpler and lesscostly power supply, compared to the resonant sinusoidal high-frequencypower supplies of the prior art inductive heating systems. In such priorart systems, an air gap provided between the heater coil and theinductively heated core constitutes a high magnetic resistance (lowpermeability) to flux, which produces a high border frequency. Toresolve this problem, the prior art systems utilize a high frequency anda high amplitude current signal in a resonant circuit, which is believedto be necessary to overcome the effects of the air gap and enable rapidinductive heating of the core.

In contrast, select embodiments of the present invention provide bettermagnetic coupling between the coil and the substrate, for example, byeliminating the air gap and more preferably embedding the coil wholly orat least partially in the substrate, and by providing a partially orsubstantially closed loop for the magnetic flux (a ferromagnetic yoke toclose the loop with the core), one or both of which can be used todecrease the border frequency of the system. This reduction of theborder frequency can then be advantageously used to provide largeramounts of energy within the harmonic current pulses above the borderfrequency of the system. This may enable use of a lower (root) frequencycurrent supply and/or without significantly increasing the root meansequence (RMS) current in the coil.

The desired current pulses are provided, in select embodiments, by alower cost power supply which includes a pulse generator supplied with alow or line frequency signal. Line frequency is typically defined as theHertz (Hz) level of power sources generally used or readily availablefor personal, commercial and industrial users, e.g. 50 or 60 Hz. Varioussignal generating or switching devices, including thyristors,gate-turn-off (GTO) thyristors, silicon controlled rectifiers (SCR), andintegrated gate bipolar transistor (IGBT) devices, can be used as thepulse generator to provide the short current pulses from a linefrequency or direct current (DC). The pulsed, nonsinusoidal currentsignal, does not require a resonant circuit; in fact it is desirable notto provide a resonant circuit so that the high frequency harmonics inthe pulses are maintained. The presence of these harmonics cansignificantly increase the power transferred inductively to the articleto be heated.

The desired current pulses may substantially improve the performance ofheating systems which utilize either a combination of inductive andresistive heating, or primarily inductive heating. The current pulsesmay be used in a system with a substantially closed magnetic loop, butthey will also improve performance in inductive heaters that do not havea closed magnetic loop. The lack of a closed magnetic loop may occur ina system having an air gap between the heater coil and the substrate, anair gap in any portion of the magnetic loop, or in a system for heatingan electrically-conductive, but non-magnetic core or load material.

The following equations illustrate a surprising improvement inperformance obtainable in select embodiments with these current pulses.Equation (1a) is used to calculate the expected resistance to the flowof eddy currents (R_(e)) in a ferromagnetic material forming a cylinder;equation (1b) is a comparable equation for a flat plate. Here it isassumed that the cylinder or plate is part of a closed magnetic loop,and a sinusoidal current is applied to a heater coil wrapped around thecylinder, or surface mounted in snake (serpentine) shape on the flatplate, at a frequency above the border frequency. For the cylinder, theequivalent resistance to the flow of eddy currents (R_(e)) is:

$\begin{matrix}{R_{e} = {\frac{\pi\; D}{L}\sqrt{\rho\mu\omega}}} & \left( {1a} \right)\end{matrix}$

where

D is the diameter of the cylinder,

L is the length of the cylinder,

ρ is the resistivity of the cylinder material,

μ is the permeability of the cylinder material, and

ω is the angular frequency of the eddy currents in the cylinder,

and for a plate:

$\begin{matrix}{R_{e} = {\frac{L}{p}\sqrt{\rho\mu\omega}}} & \left( {1b} \right)\end{matrix}$

where

L is the length of the coil conductor,

p is the perimeter of the coil conductor,

ρ is the resistivity of the flat plate material,

μ is the permeability of the flat plate material, and

ω is the angular frequency of the eddy currents in the plate,

and in both cases (cylinder and plate) where ω=2πf, f is the fundamentalfrequency, and f=1/T for a period T.

Thus, for sinusoidal currents, the equivalent eddy current resistanceR_(e) increases as the square root of the frequency ω. In contrast, ithas been experimentally determined that the equivalent eddy currentresistance may increase much faster with use of the current pulsesdescribed herein. Without limiting the scope of the invention, it may betheorized that this increased resistance is due to the effectivefrequency of such current pulses being higher than their root (nominalor fundamental) frequency, because the pulses include high frequencyharmonics. Thus, by providing current pulses with a high rate of changeof current, with respect to time, the current pulses can actually beprovided at a lower fundamental frequency than the sinusoidal currentwhich these pulses are replacing, because the steeply varying portionsof these current pulses provide high frequency harmonics that more thanmake up for their lower fundamental frequency. As a result, more powerthan expected is inductively provided to the core or load.

The desired current pulses can be generated by a variety of electronicdevices which provide rapid switching to produce much of the pulseenergy in high frequency harmonics. The use of multi-phase devices canfurther be used to boost the fundamental frequency of the pulses. Theseaspects are described in various embodiments below and with respect to acomparative experiment (see the text accompanying FIGS. 7-9).

Various implementations of an inductive heating system will now bedescribed that may advantageously utilize these current pulses.

FIGS. 1A-1D Embedded Coil, Coaxial Core/Yoke

FIGS. 1A-1B and 1C-1D show respectively two embodiments of a heatingsystem in which a heater coil is embedded in an article (ferromagneticcore and yoke) being inductively heated. In both embodiments, there isclose physical (thermal) contact and magnetic coupling between theheater coil, core and yoke.

More specifically, FIG. 1A shows a cross-sectional portion of aninductive heating system 25 which includes a ferromagnetic core 22 of agenerally cylindrical shape (disposed about center line 29) having ahollow central passage 26 through which a flowable material to be heatedis passed. For example, core 22 may be part of an extrusion die, a meltmanifold or melt conveyer, or a dynamic mixer or plasticizing unit, andthe flowable material may be any food, plastic, metal, etc., theflowable material being the ultimate target for the heat from theinductive heating system. A substantially cylindrical and coaxial outerferromagnetic yoke 28 surrounds the inner core, with substantiallydirect contact (substantial elimination of air gap) between the outerdiameter 23 of the core and the inner diameter 27 of the yoke. The outeryoke 28 closes the loop (magnetic flux lines 19) so as to retainsubstantially all of the magnetic flux within the adjacent ferromagneticcore 22 and yoke 28, thus substantially increasing the magneticcoupling, reducing the equivalent resistance to magnetic flux, anddecreasing the border frequency of the system.

A heater coil which includes a wire conductor 20 surrounded by anelectrical insulator 36, is embedded within core 22. Heater coil 20 iswrapped in a helix-shaped groove 34 around the outer diameter 23 of core22. This provides close physical contact and enables the heatresistively generated in the coil 20 to be transferred to the core 22.

Coil 20 is highly magnetically coupled to the core 22, as shown by theflux lines 19. Coil 20 can be made from a solid conductor such ascopper, or from a more highly resistive material such as nickelchromium. Core 22 is fabricated of a magnetically permeable materialsuch as iron, or other ferromagnetic material to facilitate magneticcoupling.

Coil 20 is also thermally coupled by close physical contact with core 22and yoke 28. The coil 20 is covered by a thermally-conductive,electrically-insulating material (e.g., layer or coating 36). Suitablematerials include magnesium oxide and various alumina oxides; otherelectrically insulating materials can be used as well.

A central hollow passage 26 through the ferromagnetic core 22 is definedby internal wall 24. The substance to be heated, which can be a gas,liquid, solid or some combination thereof, is positioned in (or passesthrough) the passage 26. Heat inductively generated in core 22 istransmitted to the material in passage 26 via thermal conduction and/orradiation.

Yoke 28 is made of a magnetically permeable material such as iron orsteel, or other ferromagnetic material. Yoke 28 is located adjacent toand in thermal communication with heater coil 20. Core 22 and yoke 28are in direct contact (substantial elimination of air gap) to provide aclosed magnetic loop, as well as enhanced thermal conduction. The closecoupling of coil 20 to core 22 and yoke 28, substantially reduces theborder frequency of coil 20.

A second embodiment of a similar heating system is shown in FIGS. 1C-1D.This modified system 25′ includes a modified yoke 28′ having elongatedhollow portions or slots 30, and between the slots, elongated solidportions or ribs 31; the slots 30 and ribs 31 are disposed substantiallyparallel to the core/yoke center line 29′. The slots 30 are at rightangles to the loops of coil 20 wrapped around core 22. The slots 30,which are effectively air gaps, create discontinuities or restrictionsin the eddy currents 32 within the yoke 28′, as shown in FIG. 1D (asectional view taken along section line 1D-1D in FIG. 1C). In contrast,there are no slots in core 22 restricting the eddy currents 33 in thecore 22. This arrangement results in preferential inductive heating inthe core 22, rather than the yoke 28′; this is desirable when theultimate article to be heated is a material in the passage 26 of thecore 22. Thus, a greater percentage of the power delivered to theheating system is transmitted to the article to be heated, rather thanto yoke 28′. In FIG. 1D, the current 35 in coil 20 is shown in acounterclockwise direction, and the resulting eddy current 33 in core 22in a clockwise direction. The eddy current 32 in each rib 31, betweentwo slots, is in a counterclockwise direction.

FIG. 1E Multiple Heating Zones

FIG. 1E shows a multi-temperature zone barrel extruder 12 incorporatingan inductive heating system 25 of the type previously described. Theextruder includes a barrel zone 13 with a plurality of heating zonesZ1-Z6, and a nozzle zone 14 with additional heating zones Z7-Z9. Aflowable material (to be heated) enters the barrel through an inletfunnel 16 at one end of the extruder, and proceeds through the variousheating zones 15 of the barrel and nozzle. Any one or more of theheating zones 15, such as zone Z2, may utilize the heating system 25 aspreviously described.

FIGS. 2-6 Power Supply and Switching Devices

FIG. 2 shows a power supply for providing current pulses to a heatingsystem 25′, similar to that shown in FIGS. 1A-1D. A pulse generator 40receives on input line(s) 43 a line frequency sinusoidal current signal42 of approximately 60 Hz, and generates on output line 44 currentpulses I_(c) at that line frequency, or at a multiple of the linefrequency, for delivery to coil 20. The current pulses applied to coil20 generate a rapidly changing magnetic flux that is closely coupled tocore 22 and which inductively heats core 22 (and ultimately the materialin passageway 26). Significant eddy currents are avoided in yoke 28′because of slots 30, so that yoke 28′ is not substantially inductivelyheated. By focusing the eddy currents in core 22, the overall inductiveheating efficiency is significantly improved.

The pulse generator 40 may include one or more high-speed switchingdevices, such as thyristors 48A, GTO thyristors 48B, or IGBT device 48C,as shown in FIGS. 3A-3C, respectively. These devices convert the linefrequency sinusoidal current signal 42 into current pulses I_(C), asshown in FIGS. 4A-4C, respectively.

Referring to FIG. 3A, thyristors 48A can be used for high powerapplications, e.g., in the thousands of kilowatts range. A one-phasebipolar commutator 51 (shown in a dashed box in FIG. 3A) includes a pairof oppositely oriented thyristors T1 and T2 in parallel arrangement.Circuit driver 50 provides a control signal to pins 52 to turn T1 (orT2) on when the supply line voltage is close to reversing (input signal42 crosses the horizontal axis in FIG. 4A). Once turned on, thethyristor can only turn off when the applied voltage reverses, whichhappens a short time later as shown in FIG. 4A. The period of the input60 Hz line frequency is T=( 1/60) seconds, which is approximately 17milliseconds (ms). As a result, narrow current pulses 44A are generatednear 180, 360 . . . degrees (as shown in FIG. 4A), at twice the linefrequency. The amplitude of current pulses 44A can be increased bytransformer 54 that boosts the input voltage U₀ of the line frequencysinusoidal current signal 42 to output voltage U. The RMS currentprovided in short pulses 44A is approximately equivalent to the RMScurrent directly from the line frequency sinusoidal current signal 42 ofvoltage U₀. The current pulses 44A supplied to heater coil 20(represented by R_(c), the equivalent total resistance of the heatingcoil circuit) include sharp slopes, in this case a steeply risingleading edge 46 and steeply falling trailing edge 47 (see FIG. 4A). AFourier transformation of a pulse like 44A indicates that much of theenergy of pulse 44A is in high frequency harmonics. Suitable thyristorsT1 and T2 are available from International Rectifier Corp., El Sugendo,Calif. Integrated circuit chips with drivers 50 are also available forcontrolling the thyristors.

For medium power level applications, in the hundreds of kilowatts range,a pair of oppositely oriented GTO thyristors 48B (see FIG. 3B) can besubstituted for the thyristors T1 and T2 (of FIG. 3A) to provide currentpulses 44B (see FIG. 4B) at any point in the sinusoidal input signal(see the circuit of FIG. 3B and resulting current pulses in FIG. 4B).Preferably, pulses 44B are provided at the peaks 42′ of the linefrequency sinusoidal current signal 42, as shown in FIG. 4B, reducing oreliminating the need to boost the sinusoidal current signal 42 with atransformer. Suitable GTOs are available from Dynex Semiconductor,Lincoln, United Kingdom.

For low (tens of kilowatts) and medium (hundreds of kilowatts) powerlevel applications, an integrated gate bipolar transistor (IGBT) device48C as shown in FIG. 3C can be substituted for the thyristors T1 and T2(of FIG. 3A) to provide pulses 44C having high frequency harmonics, suchas the square wave form shown in FIG. 4C. A controllable rectifier 60rectifies the line frequency sinusoidal current signal 42 of voltageU_(o) to provide a DC voltage U_(DC) which is then input to IGBT device48C. Under the direction of control circuit 62, IGBT device 48Cgenerates current pulses Ic from the rectified voltage U_(DC) to formsquare wave pulses 44C that are fed to the heater coil R_(c). SuitableIGBT devices are available from International Rectifier Corp., such asthe IRGKI140U06 device which provides hard switching at 25 KHz with avoltage over extended time of 600 volts and a current over extended timeof 140 amps. Such IGBT devices have been previously used to provide ahigh frequency signal to a resonant sinusoidal circuit for inductionheating; however in the prior resonant systems, the advantage of highfrequency harmonics in the pulses was not obtained. In contrast, herethe current pulses with their high frequency harmonics retained areprovided directly to coil 20, avoiding any use or requirement of aresonant circuit.

In each of FIGS. 3A and 3B, a parallel arrangement of two oppositelyoriented switching devices produces two pulses for each period T of thesingle-phase sinusoidal line current supply. More complex arrangementsof thyristors or GTOs can be used to provide a greater number pulses foreach period of a multi-phase supply.

One example is a three-phase, three-pulse unipolar commutator 57 shownin FIG. 5A in which a three-phase supply 59 provides three unipolarpulses to coil R_(C). In the associated timing diagram, shown in FIG.6A, the three voltage signals U_(A), U_(B), U_(C) produce three pulses44D in one period (T=( 1/60)sec≅17 ms).

Alternatively, a three-phase, six-pulse bipolar commutator 61 is shownin the circuit of FIG. 5B which produces six bipolar pulses 44E in oneperiod as shown in FIG. 6B.

In FIGS. 6A-6B, the curves U_(A), U_(B) and U_(C) denote timing diagramsof voltages in phases A, B and C.

As a further alternative, a one-phase, two-pulse unipolar pulsatorsupply 63 is shown in the circuit of FIG. 5C which produces two unipolarpulses 44F in one period, as shown in FIG. 6C.

As a still further alternative, a three-phase, six-pulse unipolarpulsator supply 65 shown in the circuit of FIG. 5D produces six unipolarpulses 44G in one period as shown in FIG. 6D.

In FIG. 6D, the curves A, B and C relate to the voltages in phases A, Band C; the curves AB, AC, BC, BA, CA, CB relate to corresponding linevoltages AB, AC and so on; in the interval 14 the thyristors 1 and 4switch on and provide the current pulse to the load R_(c) from the linevoltage AB; in the interval 1-6 the thyristors 1 and 6 switch on andprovide the current pulse to the load R_(c) from the line voltage AC,and so on.

Finally, a three-phase supply 67 is shown in the circuit of FIG. 5E,which produces 12 unipolar pulses 44H in one period as shown in FIG. 6E.In FIG. 5E, the transformers T1 and T2 provide two systems ofthree-phase voltages shifted 30 degrees; T1 is fed from 3 phases in astar-connection and T2 is fed from 3 phases in a delta-connection. InFIG. 6E, the curves AB, AC, BC, BA, CA, CB correspond to the linevoltages AB, AC and so on supplied from the transformer T1; the curvesAB′, AC′, BC′, BA′, CA′, CB′ correspond to the line voltages AB, AC andso on supplied from the transformer T2.

In each of FIGS. 5A-5E, R_(C) is the equivalent total resistance of theheating coil circuit.

Providing the additional pulses (for each period of a multi-phasesupply) increases the fundamental (root) frequency, which furthermultiplies the effect provided by the high frequency harmonic componentof the individual pulses. As such, the higher fundamental frequencyprovided by these more complex arrangements, combined with the highfrequency of the steeply varying current pulse itself, providesignificantly enhanced inductive heating.

FIGS. 7-9 Performance Comparison

An experiment was performed which illustrates an improved performance ofa combined inductive and resistive heating system powered by the currentpulses described herein, compared to the same heating system powered bya 60 Hertz sinusoidal signal voltage. FIG. 7 shows the heatingapparatus. FIG. 8 is a comparison of the heating rates. FIG. 9illustrates the shape of the current pulses used in this example.

As shown in FIG. 7, the article to be heated was a flat steel disc(core) 70, of 5 mm thickness and 160 mm diameter, covered by a flatsteel yoke 71, of 1 mm thickness and 160 mm diameter (see FIGS. 7 and7A). A heating coil 72, formed of nickel chromium rectangular wire, 2.92meters long and having a cross-section of 2.5 mm×1 mm, provided a coilresistance of 1.17 ohm. The coil 72 was covered in an insulatingmaterial 75 and was embedded in a snake-shaped groove 73 in the topsurface 74 of the disc; the coil and disc were covered by yoke 71 toprovide a closed magnetic loop. The coil 72, disc/core 70 and yoke 71,were all in close physical contact (minimizing any air gaps). From theconfiguration of FIG. 7, with the electrically insulated coil 72embedded between steel disc 70 and steel yoke 71, a border frequency wascalculated from Equation 2 (set forth below) of only 24 Hz. In contrast,a border frequency of about 2 KHz would be expected without the closedmagnetic loop (without the yoke 71).

This article was first heated with a 60 Hz sinusoidal signal (industrialpower supply). Then, after cooling to ambient temperature, the articlewas heated with current pulses from an IGBT source, similar to thatshown and described in FIG. 3C.

With a sinusoidal 60 Hz signal voltage applied across coil 72, a voltagewas measured of 9 volts RMS, thus providing a current of 10 amps RMS.The power delivered to the coil 72 and disc 70 was calculated to be 117Watts. The measured rate of change of temperature of the disc 70,plotted in FIG. 8, was 0.27° C./sec. for the 60 Hz sinusoidal voltageinput.

From an analysis of electromagnetic processes under inductive heating,and for frequencies higher than the border frequency, Kirchoff'sequation for a heater coil circuit can be represented by:U _(ps) =I _(c)(R _(c) +K _(c) ² N ² R _(e))+I _(c) jω(1−K _(c) ²)L_(c)  (2)where:

U_(ps) is the RMS voltage of the power supply source;

ω is the frequency of the power supply source above the borderfrequency;

I_(c) is the current in the heating coil (RMS);

R_(e) is the eddy current equivalent resistance;

R_(m) is the equivalent magnetic resistance of the magnetic fluxcircuit;

N is the number of turns of wire in the heating coil;

R_(c) is the resistance of the heating coil;

L_(c) is the inductance of the heating coil;

K_(c)<1 is the coefficient of electromagnetic connection between theheating coil and the eddy currents;

j=sqrt(−1) is the imaginary unit; and

where the border frequency ω_(b)=R_(m)R_(c)=2πf_(b).

For the 60 Hz sinusoidal supply signal, a total resistance of about 1.2ohms was measured from the voltage and current at the coil. The eddycurrent equivalent resistance R_(e) was calculated (from Equation 1b) tobe 0.1 ohm. Adding in the resistance of the nickel chromium wire itselfof 1.17 ohms, the total resistance expected to be measured at the coilwas 1.27 ohms. The actual measured resistance of about 1.2 ohms wasreasonably close to this expected value. It can be seen from thesenumbers that only about 8% of the power was delivered inductively usingthe 60 Hz sinusoidal supply signal. Most of the power delivered can thusbe accounted for by resistive heating of the nickel chromium wire.

In comparison, when the 60 Hz supply signal was replaced with currentpulses from an IGBT similar to that shown in FIG. 3C (obtained fromInternational Rectifier Corp., IRGP450U, rated at 500 volts and 60 ampsand hard switching to 10 KHz), current pulses with a frequency of 5 KHzwere provided. These pulses 80 from the IGBT had the profile shown inFIG. 9, with four high slope segments (81, 82, 83, 84) in each pulse,and a delay 85 between pulses. The voltage was adjusted to provide thesame current of 10 amps (as with the 60 Hz supply); however, to providea 10 amp current with the high frequency pulses provided by the IGBT,the voltage had to be increased to 114 volts. The higher voltage was theresult of the higher eddy current equivalent resistance in the heatedarticle, as transformed back to the coil. The electrical power in thecoil was now approximately 1140 Watts. The rate of temperature increasein the steel disc when utilizing these current pulses was now measuredat 2.6° C./sec, as shown in FIG. 8.

The eddy current equivalent resistance for 5 KHz current pulses wascalculated from Equation 1b, which shows that the eddy currentequivalent resistance R_(e) increases as the square root of thefrequency. With a 5 KHz frequency, which is almost 100 times higher thanthe 60 Hz line frequency, the eddy current resistance is expected to beabout 10 times higher, or about 1.8 ohms. In practice, the eddy currentequivalent resistance at 5 KHz was actually measured to be about 10 ohms(dividing 114 volts by 10 amps and subtracting the 1.17 ohms resistanceof the coil itself. The much larger equivalent eddy current resistanceactually measured shows that the eddy current resistance increased muchmore than the 10 fold increase expected from the less than 100 foldincrease in fundamental frequency. Thus, the effective frequencyincrease must have actually been much higher than 5 KHz. To account forthe almost 6 fold greater equivalent resistance, the effective frequencyincrease must have been about 180 KHz. This much higher frequency couldbe obtained because of high frequency harmonics in each of the pulses,as shown in FIG. 9.

A Fourier transform of the pulses would show the high level of energy inthese high frequency harmonics. The Fourier transform for periodicfunctions (the current pulses are periodic functions) leads to a Fourierseries:F(t)=A ₀ +A ₁ sin(ωt)+A ₂ sin(2ωt)+A ₃ sin(3ωt)+ . . .where

ω=2πf=fundamental angular frequency,

f=1/T=fundamental frequency,

t=time,

T=period of this periodic function,

A₀=constant, and

A₁, A₂, A₃, . . . =amplitudes of first, second, third, . . . harmonics.

For example, a unity square wave function F_(sw)(ωt), with fundamentalfrequency ω, has the Fourier series:F _(sw)(ωt)=4/π[sin(ωt)+⅓ sin(3ωt)+⅕ sin(5ωt)+ 1/7 sin(7ωt)+ . . . ]In the present case, the 6-fold increase in R_(e) means that about ⅚=83%of the pulse energy was in high frequency harmonics. Thus, the muchhigher than expected eddy current resistance can be explained by thepresence of these high frequency harmonics in each pulse. As a result, afar greater proportion of the power is provided to the heated article(here a metal disc) from inductive heating, rather than from resistiveheating.

In various implementations, providing greater than 15%, and moreparticularly at least 50% of the pulse energy in high frequencyharmonics would be desirable. In particular embodiments, the higher endof this range, at least 70%, may be desirable (e.g., for rapid meltingof a frozen plug in a nozzle; to allow the flow of a material through abore; or for uniform heating of an extruder barrel); a middle range of50-69% may comprise a second preference, and a lower range of 25-49% asa third preference. The operating range may vary from initial heat up toa steady state operating range.

As a basis of comparison, a rectangular shaped wave (instead ofsinusoidal and with the same amplitude) has about 25% of its energy inhigh harmonics, while a triangular shaped wave (with the same amplitude)has about 10%.

In select embodiments described herein, where it is desired to utilizeboth inductive and resistive heating, the heating power which isconsumed from the power supply includes two portions:

a) power of the resistive heatingP _(R) =I _(C) ² R _(c)

b) power of the inductive heatingP _(I) =I _(c) ² K _(c) ² N ² R _(e)where I_(c) is the current in the heater coil (RMS); R_(c) is theresistance of the heater coil; R_(e) is the equivalent eddy currentresistance; N is the number of coil turns; and K_(c) is a coefficient ofelectromagnetic connection between the heating coil and the eddycurrents. In the combined resistive/inductive implementations describedherein, the resistive component P_(R) will contribute to the overallheating efficiency when this heat is transferred to the article to beheated, as compared to prior art systems which cool the heater coil andthus lose this resistive component of the heating power. Where theheating coil is embedded in the heated article, the coefficient ofelectromagnetic connection is increased almost to K_(c)=1, whichincreases the induction portion of the heating power P_(I) under thesame coil current. With I_(c) (a maximum allowed current for a givencoil), N and K_(c) fixed, the inductive component of the heating powerP_(I) is increased by increasing the eddy current equivalent resistanceR_(e) (as previously described with respect to Equation 1).

An analysis of electromagnetic processes of inductive heating under anarbitrary input current (not necessarily sinusoidal variation),indicates that the eddy current resistance R_(e) is a function of therate of change of current in the coil. The experimental data suggeststhat:R _(e)˜(dI _(c) /dt)^(n)where n>1, I_(c) is the current in the coil, and t is time. In view ofthis relationship, the proportion of heating from inductive heating canbe significantly increased, without increasing the current in the coil,by replacing a high fundamental frequency sinusoidal current supply,with current pulses having steeply varying portions. These pulses can beprovided at a lower fundamental frequency than the sinusoidal currentthey are replacing, because the steeply varying portions of the currentpulses provide harmonics that more than make up for the lowerfundamental frequency.

In select embodiments, with better coupling provided by eliminating theair gap, embedding the coil in the substrate, and/or providing a yoke toensure a closed loop for magnetic flux, the border frequency can bedecreased. This facilitates an improvement in inductive heatingperformance by providing current pulses with high frequency harmonicsabove the border frequency.

As described above, a lower cost power supply for induction heating canbe provided, which includes a pulse generator excited with a low or linefrequency. Signal generating devices, including thyristors, GTOs, andIGBT devices can be used to provide short current pulses from the linefrequency or direct current. The high frequency harmonics in thesecurrent pulses are preserved (in the absence of a resonant circuit) toincrease the power transfer to the inductively heated object. Also,cooling of the heater coil may not be required, in contrast to priorsystems.

An IGBT device, capable of higher voltage and current than used in thedescribed experiment, can be run at a frequency higher than 5 KHz, thusproviding more power to the load for heating with the same coil and withthe same current in the coil as provided in the 60 Hz experiment.

FIG. 10 Furnace

FIG. 10 illustrates a furnace 90 as an alternative heating system. Thefurnace includes a bowl-shaped container 91 (as a ferromagnetic core),having a bottom wall 97 and an upwardly flared side wall 98, and a coil93 embedded in a cubical groove wrapped around the outer circumferenceof the side wall. A sleeve-like yoke 92 covers the core side wall 98, indirect contact, closing the magnetic loop. A fixed or removable lid 94covers the top opening of the container 91. A material 95, which ismolten or otherwise desired to be maintained at a select temperature, iscontained within the core 91. A detail section in FIG. 10A shows thecoil 93, surrounded by an insulating layer 96, in close contact with thecore side wall 98 and yoke 92.

FIG. 11 Water Heater or Chemical Reactor

FIG. 11 shoes a water heater or chemical reactor 100 implementation inwhich a cylindrical core 101 has an embedded coil 103 in its outersurface, and a cylindrical yoke 102 surrounds the core but is separatedtherefrom by an air gap 107. A disc-shaped lower yoke 105 in directcontact with yoke 102 closes the bottom end of the heater/reactor, and adisc-shaped upper yoke 104 in direct contact with yoke 102 closes thetop end of the heater/reactor, thus closing the magnetic loop. The closephysical (direct) contact between the ferromagnetic core 101 with theupper and lower ferromagnetic yokes 104 105, and the ferromagnetic sidewall yoke 102, enhances the coupling of the closed magnetic loop. Aflowable material to be heated may be sent through the central passage109 in the heater/reactor.

FIG. 12 Heater Patches

FIG. 12 shows a further alternative heating system 110 in which achemical container or reactor 112 has heater patches mounted thereon.Two alternative types of heater patches are shown, circular discs 114 onthe right, and square or rectangular plates 116 on the left. Theconstruction of these heater patches may be similar to the structure ofFIG. 7.

FIGS. 13-19 Layered Heating Structures

FIG. 13 is a schematic cross section of a heating apparatus 120including an outer layer 122, an inner ferromagnetic layer 126, aheating element 124 disposed between the outer layer and inner layer,and an article to be heated 128 adjacent to the inner ferromagneticlayer. The heating element is surrounded with electrical insulation 125,to isolate the electrical conductor from the inner ferromagnetic layerand the outer layer. The outer layer may any of various materials,including ferromagnetic or non-ferromagnetic materials, electricallyconductive or non-conductive materials, and thermally insulating ornon-insulating materials. The outer layer may also be wholly or partlyan air gap.

FIG. 14 shows an alternative apparatus 130 including an outer layer 132,an inner ferromagnetic layer 136, a heating element (with conductor 134and insulating cover 135) disposed between the outer and inner layers,and an article to be heated 138 adjacent to the inner ferromagneticlayer. Here, the inner ferromagnetic layer has a thickness A ofapproximately 3*. Delta (*) is the depth of penetration of the inducededdy current in the ferromagnetic material of inner layer 136. Thisthickness A provides a preferred efficiency of transmission of heat fromthe ferromagnetic layer 136 to the article/material 138 to be heated.

FIG. 15 shows an alternative apparatus 140 including an outer layer 142,an inner ferromagnetic layer 146/149 and a heating element (conductor144 with insulating cover 145) disposed between the inner and outerlayers. Again, the article 148 to be heated is disposed adjacent to theinner ferromagnetic layer. Here, the inner layer 146/149 includescooling passages 147 through which a cooling medium may be passed (e.g.,intermittently) to reduce the temperature of the inner layer 146/149, asdesired. Alternatively, the cooling passages can be placed in the outerlayer 142, or they may be provided in both the outer and inner layers.

FIG. 16 shows a further alternative apparatus 150, including an outerlayer 152, an inner ferromagnetic layer 156, and a heating element(conductor 154 with insulating covering 155) disposed between the innerand outer layers. Again, the article 158 to be heated is disposedadjacent to the inner ferromagnetic layer 156. In this embodiment, theheating element is a hollow rectangular electrically conductive heatingelement 154 with an interior cooling passage 157. A cooling medium canbe passed through the central cooling passage for cooling (e.g.,intermittent) of the inner layer 156 and/or outer layer 152.

FIG. 17 shows another apparatus 160 including ferromagnetic outer andferromagnetic inner layers 162, 166, respectively, surrounding a heatingelement (conductor 164 with interior cooling passage 167 and insulativecovering 165). The inner layer 166 includes a corrosion-resistant andthermally conductive layer 169 adjacent to the article to be heated 168.The inner and outer ferromagnetic layers 162, 166 form a substantiallyclosed magnetic loop for the induced magnetic field. The outer layer 162may be thermally insulative.

A thermal spray (TS) method may be used to manufacture an integratedlayered heating element in the various structures described above inFIGS. 13-17, and below in FIGS. 18-19. These integrated layeredstructures enable the heating element to be part of a structural elementof the apparatus, able to withstand compressive forces applied, forexample, to a melt channel, blow mold, or compression molding system.

FIGS. 18-18A shows for example a layered heating apparatus whichincludes a heating element disposed between an outer layer 172 and aninner ferromagnetic layer 176. The heating element has an outerdielectric layer 175A, an interior cooling passage 177, a conductivecoil layer 174 and an inner dielectric layer 175B. Alternatively, thecooling passage can be eliminated from within the heating element or itmay be provided at other locations in the heating apparatus. Tomanufacture this structure, channels 171 are formed in the outer surface173 of the inner layer 176. The inner dielectric layer 175B is thermallysprayed in the channels 171. Next, the electrically conductive coillayer 174 is thermally sprayed over the inner dielectric layer 175B. Theapplied layers 175B and 174 may be removed from outer surface 173 exceptin the channels 171. The second dielectric layer 175A may be thermallysprayed on the inner surface of outer layer 172. The outer and innersections are then joined together, leaving a cooling passage 177 betweenlayers 175A and 174.

An alternative thermal-sprayed implementation is shown in FIGS. 19-19A.Here, the apparatus 180 includes an outer non-ferromagnetic mold base182, an outer dielectric insulating thermal-sprayed layer 185A, aheating coil layer 184, an inner dielectric thermal-sprayed layer 185B,an inner ferromagnetic molding surface layer 186, and an article to beheated 188 disposed adjacent the inner molding surface. An appliedmagnetic field induces an eddy current in the inner ferromagneticmolding surface layer 186. The substantially non-ferromagnetic (e.g.,aluminum) mold base 182 will not have significant induced eddy currents,compared to the inner layer 186, and thus will be substantially cooler.Once the heating element 184 is turned off, rapid cooling of the moldingsurface layer 186 can take place. The substantially greater mass of thecooler outer mold base 182 will pull heat out of the substantially lowermass molding surface layer 186. Here, the cooling medium or mechanism isthe outer non-ferromagnetic mold base 182 itself, acting as a heat sink.

FIGS. 20-22 Injection Nozzle

In a traditional nozzle heating assembly, resistive heater bands arelocated on the outer circumference of the injection nozzle. Heatresistively generated in the heater bands must then be thermallyconducted from the outer surface of the nozzle to its the inner surface,where a material (to be heated) flows through a central nozzle passage.This is a relatively inefficient method of heating, and it is difficultto provide either a uniform temperature or rapid heating. If the nozzleis heated too rapidly, thermal gradients are produced which may lead tostructural failure (e.g., cracks) in the nozzle. The nozzle itself is anextension of an extruder/barrel apparatus, and typically is subjected toseveral tons of force, e.g. 5-10% of the clamp tonnage. Thus, smallcracks induced by excessive thermal gradients are likely to grow andlead to ultimate failure.

Also in the prior art design, a separate cooling circuit is provided inthe injection nozzle to prevent “drool” or “stringing’ of the plasticmelt when the mold is opened for removal of the molded object. Thus,every molding cycle, the movable side of the injection mold is openedand, for the duration of the mold disengagement, the flow of moltenplastic to the nozzle must stop. If drool or stringing occurs from theseparated melt passages (in the mold and nozzle), it must be removed,causing down time and a loss of material. Alternative methods to controlthis problem are expensive, or in many cases not practical.Decompression of the extruder or mechanical shut-off of the nozzle mayhelp prevent drool, but certain molding materials do not allow fordecompression because it creates defects (air inclusion) in the moldedpart. Mechanical shut-off devices are problematic because they requireextra moving components, electrical sensors, hydraulic hoses (with anaccompanying risk of hydraulic leak and fire), wear of components,accurate fit of shutoff pins, and maintenance.

Thus, it would be desirable to control a melt passage orificetemperature at a precise level and/or allow rapid heating and cooling ofthe orifice. This could reduce or eliminate the need for mechanicalshut-off devices. Further, such thermal control of the melt would enableformation of a solidified segment, partially solidified segment, or anincrease of viscosity of the melt such that it does not drool or string.

It would also be desirable to provide a more compact and energyefficient heating apparatus, compared to the traditional resistiveheater bands applied to the outer surface of the nozzle. With theseprior known devices, heating and cooling are applied far away from thedesired area (central passage) to be heated or cooled, thus resulting inpoor thermal response time. As a result, the heating and coolinghardware are increased in size to compensate for the thermalinefficiencies, making the heating and cooling apparatus very bulky.Still further, the lifetime of a resistive heater, at temperatures suchas 600° F., is very limited, increasing the down time when the heaterneeds to be replaced.

FIGS. 20-22 illustrate an improved heating and cooling system for aninjection nozzle assembly. The nozzle assembly 200 is generallycylindrical, having a central through passage 208 extending from a firstor barrel/extruder end 210 and to a second mold end 212. The nozzleincludes an inner component 202 and a coaxial outer component 204, and acoiled heater element 206 disposed between the inner and outercomponents. The heating element 206 is shaped as a helical coil wrappedaround the outer cylindrical surface 213 of tube 214, on the mold end ofthe inner nozzle component 202. The tube and heating element fit withinan inner bore 216 of the outer component 204, as shown in FIG. 21. Acontinuous through passage is formed by a central bore 220 of the outernozzle 204 at the mold end 212, and continues through a central passage222 extending the length of the inner nozzle 202 to the barrel/extruderend 210.

A plastic melt passes through the central passage, coming from anextruder, through the nozzle 200, through a hot runner system, and intothe mold. Following injection of a predetermined amount of plastic meltfrom nozzle 200 into the mold, and following some cooling time in themold, the mold is opened, i.e., separated from the nozzle, at which timethe flow of plastic melt through the nozzle must cease. The heating andcooling elements of the present nozzle enable an energy efficient andrelatively simple mechanism for controlling that melt flow during theinjection cycle.

During a first portion of the injection cycle, molten plastic will flowthrough the central passage 208 of the heated nozzle assembly. A currentpulse signal is applied to the heating element 206, which generates analternating magnetic field. This field generates an induced eddy currentin the ferromagnetic tube 214 of the inner nozzle, heating the innernozzle tube. Heat in the inner nozzle tube is transmitted to the moltenplastic flowing through the central passage 222 of the inner nozzle. Theheating element 216 is positioned relatively close to the centralpassage 222, compared to the prior art resistive heating bands appliedon the outer surface of the nozzle assembly.

In the embodiment shown, the heater coil is a nickel chromium alloy(NiCr) coiled element having a relatively large cross section to reducethe amount of resistive heat generated in the coil. The coil is coveredby an electrically insulating material in order to electrically isolatethe heating element from the inner and outer nozzle components 202,204.Furthermore, a passage 230 is formed between the inner and outer nozzlecomponents through which a cooling medium can be passed. During a secondportion of the injection molding cycle, the current pulse signal can bepartially or totally reduced, to reduce or eliminate the inductiveheating generated in the ferromagnetic inner nozzle 202 and thus reducethe heat transmitted to the molten plastic in the passage 222. To coolthe ferromagnetic inner nozzle 202, a cooling medium is passed throughthe cooling passage 230 in order to draw heat out of the inner nozzletube 214. This enables rapid cooling of the plastic melt during thesecond portion of the cycle. The mold can then be opened and plasticwill no longer flow through the nozzle, due to formation of a solidifiedsegment, partially solidified segment, or increased viscosity of theplastic.

The outer nozzle 204 need not be formed of a ferromagnetic material,where inductive heating of the outer nozzle is not desired.Alternatively, the outer nozzle can be made of a ferromagnetic materialwhere it is desired to inductively heat the outer nozzle as well as theinner nozzle.

This nozzle design enables rapid heating to achieve a uniform or steadystate heating, as well as rapid cooling during another portion of theinjection cycle. At relatively low temperature applications, it ispossible to use a copper heating coil 206 without any cooling period.However, for higher temperature applications, e.g. 600° F., a coppercoil would oxidize and incinerate within a short period of time. Inhigher temperature applications it is preferred to use a Nichrome (NiCr)coil which can withstand higher temperatures.

Furthermore, this implementation provides a compact and efficient nozzledesign. Heat is generated closer to the central passage 222, where it istransmitted to the material to be heated. Cooling is also applied closerto the inner nozzle, to enable rapid cooling of the inner nozzle andmelt during the mold open (disengagement) portion of the injectioncycle.

FIGS. 23-25 Multi-Zone Hot Runner Nozzle

FIGS. 23-25 show another embodiment of a heating apparatus incorporatedin a multi-temperature zone nozzle assembly 240. A steel hot runnernozzle 242 has an elongated cylindrical portion 244 with a centralpassage 246. A heater sleeve assembly 247 includes a heating coil 248disposed on a tube or sleeve 250 and covered by outer layer 254. Theelectrically conductive coil 248 is provided in a serpentine pattern onan outer surface 252 of the inner sleeve 250. The sleeve assembly 247slides over cylinder 244 of the nozzle 242. The inner sleeve 250 andouter layer 254 provide electrical insulation between the electricalconductor 248 and the ferromagnetic steel nozzle cylinder 244. In thisembodiment, which relies primarily on inductive heating of theferromagnetic steel nozzle, it is not required to have intimate physicalcontact between the heating assembly and nozzle (as would be requiredfor transfer of resistive heat generated by the electrical conductor).

FIG. 23 shows an exploded view of the multi-zone heating assembly forthe injection molding nozzle. The outer layer 254 is shown removed fromthe assembly, but in practice it is permanently attached over theheating element and inner sleeve 250. Two temperature control zones(Zone 1 and Zone 2) are shown, illustrated by an upper electricalconductor pattern 256 having fairly close spacing between adjacentelements in the serpentine pattern, and a lower more widely spacedpattern 258. If the upper and lower conductive patterns are powered bythe same signal, the upper pattern 256 would deliver more heat than themore widely spaced lower pattern 258. The use of the serpentine patternallows for rear exiting of the leads 260 from the multiple zones; anelectrical connecter 262 is provided at the lower end of the innersleeve.

FIG. 24 is a profile view of the assembly with the heater sleeveinstalled over the nozzle 242. Various lengths and numbers of zones canbe implemented to accommodate different nozzle lengths. FIGS. 25 and 25Ashow cross-sectional views depicting the heating element 248 disposedbetween the inner dielectric (e.g., ceramic) tube 250 and the outerdielectric (e.g., ceramic) layer 254.

A benefit of this nozzle and heater sleeve assembly is the ability torapidly remove the heater sleeve assembly 247 from the nozzle 242 inorder to clean or otherwise service the nozzle. In contrast, priorheating elements required a close tolerance fit to the nozzle makingservice much more difficult and time-consuming. For example, if aresistive heating element fails and needs to be replaced, it often mustbe pried loose from the nozzle. Here, the relatively loose fittingceramic sleeve can provide, for example, up to a half millimeter gapbetween the inner sleeve 250 and nozzle cylinder 244, and still provideeffective inductive heating of the nozzle. Also, the heater sleeveassembly can be economically manufactured by providing an inner ceramictube 250, spraying the heating element 248 over the outer surface of thetube 250, and then casting the outer ceramic layer 254 over the heatingelement 248 and tube 250. The cast outer layer 254 may provide themajority of the structural integrity of the sleeve.

FIGS. 26-29 Blow Mold

FIGS. 26-30 show another embodiment in which a heating apparatus isincorporated into a container blow molding apparatus 300. In thisexample, a heating element will provide primarily inductive heating inorder to rapidly heat a thin film of ferromagnetic material (a moldinsert) provided on the inner surface of the mold. The outer mold canthen be maintained at a lower temperature. Rapid heating and cooling ofthis thin ferromagnetic molding surface layer enables a reduction in theoverall cycle time for blow molding and/or thermally conditioning of acontainer.

Conventional thermal conditioning processes utilize a high moldtemperature to condition a plastic container within the blow mold. Thishigh mold temperature requires the use of air cooling, on the internalsurface of the blown container, in order to permit removal of thecontainer from the mold without excessive shrinkage or distortion. Theseconventional molds may have surface temperatures of 260-280° F., andrequire the constant introduction and exhausting of compressed air atpressures of 600 psi (40 bars), to cool the internal surface of thecontainer while the outer surface is in contact with the hot mold.Depending upon the polymer used, this type of thermal processing may beused to provide increased levels of crystallinity.

The use of high mold temperatures and internal air flushing/coolingreduces the throughput, compared to a lower operating temperature mold.For example, at a lower mold temperature of 190° F., a bottlemanufacturer may be able to produce 1400 containers per mold per hour;in contrast, at a higher mold temperature of 260-280° F., this numbermay be reduced to 1200 bottles per hour or less. This reduction inprocess throughput is a significant cost disadvantage, in addition tothe greater cost and complexity of the molding apparatus required by aircooling.

FIGS. 26-27 show one half of applicant's blow molding apparatus 300 formaking a plastic bottle 290. In the exploded view of FIG. 27, an outermold portion 306 made of aluminum Al has an inner shaped contour 308 forforming one half of the container sidewall. A serpentine groove 310 isprovided in this inner mold surface, which is shaped to accommodate aheating coil 302 and an adjacent outer dielectric coil 312. The heatingcoil 302 is disposed between this outer dielectric coil 312, which isalso provided in a serpentine pattern, and an inner dielectric layer314, shown as a continuous sheet. Adjacent the opposite side of innerdielectric layer 314 is a relatively thin layer (compared to the outermold 306) ferromagnetic mold insert 304, here for example made of NiCr,and formed as a continuous sheet. The mold insert 304 has an outersurface 318 in contact with the inner dielectric layer 314 and has as aninner surface 320 a shaped contour with details to be reproduced in theblown sidewall 292. The outer mold base 306 is provided with grooves 322and an electrical connector 324 for leads supplying current to theheating coil 302.

FIG. 28 is a schematic sectional view illustrating the mold surfaceheating of the bottle wall. The heating coil 302, with surroundingdielectric layers 312, 314, is shown disposed between the outer moldbase 306 and the much thinner mold insert 304. The current in heatercoil 302 generates a magnetic flux 301 which extends through the NiCrmold insert 304 and also the Al mold base 306. Because the Al mold base306 has much lower resistance to eddy currents compared to the NiCr moldinsert 304, the mold base 306 will not generate significant heat fromeddy currents. In contrast, the mold insert 304 will generatesignificant eddy currents that inductively heat the mold insert 304. Thebottle sidewall 292 is held in close contact with the mold insert 304during the molding process and thus heat is transferred from theinductively heated mold insert 304 to the bottle wall 292.

FIG. 29 illustrates a reduction in cycle time that can be achieved byuse of this blow mold apparatus. The steps of the blow molding cycle 340are listed on the left side of the graph; the horizontal axis is time,in seconds (sec). The dotted line 342 represents the temperature of thebottle sidewall, according to a temperature scale 344 provided on theright side of the graph. Also provided on the far right is anapproximate indication in seconds (duration 346) for each portion of thecycle.

At the beginning of a new cycle, a heated preform (from which the bottlewill be formed) is inserted into the mold (duration 0.1 sec). Thepreform enters the mold at a temperature of about 190° F., about thesame as that of the outer mold base. The mold insert is being heated tothe desired maximum temperature, namely 280° F. Inductive heating of themold insert will continue for 1.5 seconds of the cycle. The mold isclosed (at t=0.1 sec in the cycle), and stretch blow molding of thepreform is initiated (duration 0.2 seconds). The expanded preformcontainer contacts the heated mold insert, and the sidewall temperaturecontinues to rise until it reaches the mold insert temperature of 280°F. (at about t=0.7 sec in the cycle). The pressure is held in order tomaintain the bottle sidewall in contact with the mold for purposes ofthermal conditioning (duration 1.2 seconds; from t=0.3 to 1.5 sec in thecycle). At this point in the thermal conditioning, the inductive heatingis reduced or turned off and cooling of the mold insert begins. Thelower temperature outer mold base now draws heat from the mold insert,and as a result the bottle wall, still in contact with the mold insert,drops in temperature (from t=1.5 to 1.9 sec in the cycle). Next, as theblow pressure is exhausted (at t=1.9 sec), the cooled bottle wallreaches a temperature acceptable for ejection from the mold. The mold isopened (at t=2.1 sec) and the container is ejected (at t=2.3 sec). Oncethe part is ejected, heating of the mold insert begins again for thenext cycle. The desired maximum mold insert temperature is reached andthe next preform is inserted to begin a new cycle. The total cycle time(from insertion of the perform to ejection of the container), whichincludes heating, expanding and conditioning the container over atemperature range from 190° F. up to 280° F., is abut 2.4 sec.

In this example, inductive heating enables rapid heating of the thinfilm ferromagnetic molding surface (i.e., the mold insert 304). Byterminating (or substantially reducing) power to the heating coil 302,the thin ferromagnetic film quickly cools to the lower outer moldtemperature (190° F. of mold base 306); this eliminates the need forinternal air circulation of the blown container. There are considerablecapital, energy and maintenance savings by eliminating the need forinternal air cooling of the container. The rapid thermal cycling of theblow molding surface may provide a container having improved propertiesfrom thermal conditioning, such as sharper detail in the sidewall and/ora stiffer feel to the container. This is achieved without the need forthe slower throughput of a higher mold base temperature, and without thecosts associated with internal air cooling.

This apparatus and method may also provide significant benefits over theprior art two-mold process used to produce containers with high levelsof thermal conditioning. In the two-mold process, a container is blownin a first mold, removed and subjected to radiant heat in a conditioningoven, and then transferred to a second blow mold and reblown into afinal desired shape. The resulting containers are typically used forvery high use temperatures (e.g., pasteurization applications). By useof the present inductive heating elements, and molding over a longercycle time, the required high crystallization levels can be achievedutilizing a single mold process. This significantly reduces the capitaland operating cost requirements. As a still further alternative, it ispossible to incorporate ferromagnetic additives to the polymer fromwhich the bottle is blown in order to inductively heat the bottle walldirectly, as well as by a transfer of heat from the molding insert.

FIGS. 30-36 Compression Mold

FIGS. 30-36 illustrate another embodiment in which a heating apparatusis incorporated into a compression mold 400. The closed compression moldis shown in FIG. 30. FIG. 31 is an exploded sectional view showing thevarious parts of the compression mold, which include in serial orderfrom top to bottom:

-   -   core 402;    -   ring 404;    -   mold insert 406;    -   upper dielectric layer 408;    -   heating element 410;    -   lower dielectric layer 412;    -   heater plate 414;    -   cooling plate 416;    -   insulating board 418; and    -   backing plate 420.

The assembled components are shown in cross section in FIG. 33. FIGS.34-35 are enlarged sectional views.

A schematic partial cross section in FIG. 32 can be used to describe theinductive heating of the mold insert 406 and heater plate 414, andsubsequent cooling of the heater plate 414. In FIG. 32, the insulatingboard 418 (now shown on top) provides thermal insulation. The next layeris the cooling plate 416 in which cooling passages 422 are provided forintermittent cooling, as described in the process below. The next layeris the ferromagnetic heater plate 414, and below that the mold insert406 and molded part 430. The electrically-conductive element 410 isdisposed in grooves 424 within the heater plate 414. A current in coil410 generates a magnetic flux which induces an eddy current in bothheater plate 414 and the adjacent mold insert 406. The heat generated inthe mold insert 406 is then transferred to the adjacent article 430(being molded in ring 404, between core 402 and mold insert 406). Inthis example, the article 430 is a bipolar plate or fuel cell. More heatwill be transferred from the mold insert 406 to the adjacent article430, than transferred to the heater plate 414. The cooling channels 422in cooling plate 416 allow for intermittent cooling of the heater plate414 (arrows 430 show heat being drawn from plate 414). In alternativeembodiments, only one of heater plate 414 and mold insert 406 isferromagnetic.

According to one method embodiment described in FIG. 36, the apparatusshown in FIGS. 30-35 can be used as follows. The steps 440 of the methodare shown in the leftmost column 440 of FIG. 36; the horizontal axis istime in seconds (sec). The dotted line 442 represents the temperature ofthe heater plate 414, according to a temperature scale 444 on the rightside of the graph. The duration 446 of each method step is provided onthe far right side of the graph. During this molding cycle, the changein mold temperature is 200° F., going from 230 to 430° F. The goal is toheat and cool the heater plate 414 within the shortest time, in order toreduce the overall cycle time.

In a first step at the start of a new cycle, the mold insert 406 hasbeen heated to a maximum temperature of 430° F. During an initial 30seconds of the cycle, the temperature of the heater plate 414 increasesfrom 230 to 430° F. During a latter portion of this heat-up step, themolding material is loaded into the mold (at t=20-25 sec of the cycle).When the mold surface temperature reaches the high temperature of 430°F., the mold can be closed and compression applied (at t=25-30 sec ofthe cycle). During a hold and cure stage (t=30-70 sec), the moldtemperature is maintained at 430° F. After holding the molded articlefor 40 seconds at 430° F., the cooling portion of the cycle begins. Acooling medium is applied (at t=70 sec) to the cooling channels in themold base, and heat is drawn out of the heater plate 414, andconsequently out of mold insert 406 and the molded article 430. Thetemperature of heater plate 414 steadily drops (from t=70-115 sec) untilthe mold can be opened (after 45 seconds) and the part ejected (at t=115sec). The temperature of the mold insert is now 230° F. The coolingchannels are then purged of cooling fluid (from t=115-120 sec) so thatthe heater plate 414 is at its low temperature of 230° F.; at the sametime, heating element 410 is turned on to resume heating of the moldinsert 406. This last step takes about 5 seconds. The overall cycle timeis about 2 minutes.

Additional Embodiments and Alternatives

The heater coil may be any type of material or element that iselectrically conductive (with varying levels of resistivity) forpurposes of generating an alternating magnetic field when supplied withan alternating electric current. It is not limited to any particularform (e.g., wire, strand, coil, thick or thin film, pen or screenprinting, thermal spray, chemical or physical vapor deposition, wafer orotherwise), nor to any particular shape.

A nickel chromium heater coil is described in one or moreimplementations herein, as being a substantially more resistive materialthan copper. Other “resistive conductor” heater coil materials includefor example alloys of nickel, tungsten, chromium, aluminum, iron,copper, etc.

The article being heated can be any object, substrate or material (gas,liquid, solid or combination thereof which is wholly or partlyferromagnetic and itself can be inductively heated by the application ofa magnetic flux to induce eddy currents therein, or which receives heatby transfer from another article that is directly or indirectly beinginductively heated. There is no restriction on the geometry, dimensionsand/or physical location of the article with respect to the heater coil.

The article which undergoes inductive heating is not limited to a singlearticle, e.g., a magnetic core as described in certain embodiments, butmay include multiple articles. In addition to (or instead of) a core asthe heated article, the ultimate material to be heated may be anelectrically conductive material (such as aluminum or magnesium) passingthrough a flow passage in the core. The material in the flow passage canitself be heated by induction and/or by transfer of heat from the core.

A slotted yoke is described as one implementation of an article whichcloses the magnetic flux loop (with the core), but is less efficient interms of inductive heating because the slots (essentially air gaps)create discontinuities or restrictions in the magnetic field. Many otherstructures can be used to create such discontinuities or restrictions,for example, portions of the yoke can be made of materials (other thanair) which are not magnetically permeable or substantially lesspermeable, than the ferromagnetic core, or the yoke can be made fromferrite, fluxtron or similar materials with high resistivity to the flowof eddy currents. Also, yoke is used broadly and is not limited to aspecific structure, shape or material.

The heater coil may be formed in a serpentine pattern disposed on oradjacent a surface of the article and provide a magnetic field inalternating directions (with respect to position) across the article.The heater coil may be formed in a cylindrical pattern wrapped around athree dimensional article and provide a magnetic field in the samedirection (with respect to position) inside the coil. In variousembodiments, the electrical conductor can be a hollow element or a solidelement and it can take various shapes and forms, such as spiral,serpentine, looped spiral or looped serpentine. One benefit of a loopedspiral or looped serpentine element is that both electrical leads canexit at the same location. The conductive coils can have a variablepitch (distance between coils), which will affect the resulting magneticfield generation. Depending on available space and desired heatingpower, the shape and distance between coils can be varied to vary theheating power density. A description of basic heater coil designs isfound in S. Zinn and S. L. Semiaten, “Coil Design and Fabrication,” a 3part article, published in Heat Treating, June, August and October 1988.

The heating output of the coil is a function of the frequency, currentand number of turns of the heating element. This correlation can bedescribed as:

${I^{2}N^{2}\sqrt{\omega}} = {\alpha\; P_{req}}$where α is a function of the material and geometry.

I=current

N=number of turns

Q=frequency of power source

P_(req)=power required to heat material

The heating and cooling channel configurations can be varied to obtain adesired heating profile or pattern for speed, uniformity and efficiency.

FIGS. 37-46 illustrate how different induction fields are created bydifferent configurations of the heater coil (heating elements). Theexamples include a coil shaped in the form of: a cylindrical helicalcoil (FIGS. 37-38); a planar spiral coil (FIGS. 39-40); a planar loopedspiral coil (FIGS. 41-42); a planar serpentine coil (FIGS. 43-44) and aplanar looped serpentine coil (FIGS. 45-46). A perspective view isprovided, as well as a cross section. The magnetic flux is shown byarrows, the coil current by I_(c) and the eddy current by I_(e).

The following formulas can be used, for example, to calculate theinductive power P_(I) produced by a coil having the respective shape:

Spiral:

$P_{I} = {\frac{\pi^{3}}{16}R^{2}j_{gen}^{2}{d^{2}\left\lbrack \frac{\mathbb{d}}{\mathbb{d}{+ \Delta_{1}}} \right\rbrack}\sqrt{\mu\rho\omega}}$

Serpentine:

$P_{I} = {\frac{\pi^{2}}{16}R^{2}j_{gen}^{2}{d^{2}\left\lbrack \frac{\mathbb{d}}{\mathbb{d}{+ \Delta_{1}}} \right\rbrack}\sqrt{\mu\rho\omega}}$

Looped Spiral:

$P_{I} = {\frac{\pi 3}{16}R^{2}j_{gen}^{2}{d^{2}\left\lbrack \frac{\mathbb{d}}{\mathbb{d}{+ \Delta_{1}}} \right\rbrack}\sqrt{\mu\rho\omega}}$where

-   -   R=eddy current resistance of ferromagnetic load    -   j_(gen)=current density in the coil    -   d=diameter of the coil    -   Δ₁=distance between coils    -   μ=magnetic permeability of ferromagnetic load    -   ρ=resistivity of ferromagnetic load    -   ω=angular frequency of eddy currents in load

FIGS. 37-38 illustrate a cylindrical helical coil 502 wrapped around asolid cylindrical ferromagnetic core 504. FIG. 38 shows in cross-sectionan upper row of coil elements 506, for which the current direction I_(c)is into the page, generating a magnetic flux 508 in a clockwisedirection in an upper portion 510 of the core, which generates an eddycurrent I_(e) coming out of the page in the upper portion of the core.In a lower set of coil elements 512, the current is coming out of thepage, generating a magnetic field 514 in a clockwise direction in alower portion 516 of the core and generating an eddy current I_(e) goinginto the page in the lower portion of the core. The upper and lowermagnetic fields 508, 514 reinforce each other in the core 504.

FIGS. 39-40 illustrate a planar spiral coil 522 mounted on an uppersurface 524 a flat ferromagnetic article 526. FIG. 40 shows incross-section a left set 530 and a right set 536 of coil elementsadjacent to upper surface 524 of the article. The left set 530 of coilelements have current I_(c) going into the page, and generate acounter-clockwise magnetic field 532 in the left-portion 534 of thearticle, with an eddy current I_(e) coming out of the page. Thedirections are reversed for the coil current I_(c), magnetic field 538,and eddy current I_(e) on the right portion 540 of the article.

FIGS. 41-42 illustrate a planar looped spiral coil 550 mounted on anupper surface 552 of a flat ferromagnetic article 556. Four adjacentcoil sections in the looped spiral are identified as A, B, C, and D.FIG. 42 shows in cross-section the directions of the respective coilcurrent I_(c), magnetic field 558, and eddy current I_(e) for each ofthe four identified coil sections.

FIGS. 43-44 illustrate a planar serpentine coil 570 mounted on an uppersurface 572 of a flat ferromagnetic article 574. Four adjacent coilsections in the serpentine coil are labeled A, B, C, and D. FIG. 44shows in cross-section the directions of the respective coil currentI_(c), magnetic field 578, and eddy current I_(e) for the four coilsections.

FIGS. 45-46 illustrate a planar looped serpentine coil 580 mounted on anupper surface 582 of a flat ferromagnetic article 584. Four adjacentcoil sections are labeled A, B, C, and D. FIG. 46 shows in cross sectionthe directions of the coil current, I_(c), magnetic field 588, and eddycurrent I_(e) for the four coil sections.

The RMS current in the coil and power provided by the coil can becontrolled by varying the period (the fundamental frequency) of thepulses at constant pulse width, or by varying the width of the pulses atthe constant fundamental frequency of the pulses provided to the coil,or by both.

By fundamental frequency it is meant the frequency of a pulserepetition. Each pulse may contain multiple sloping portions or steepedges (the harmonic portions), but between each pulse is a relativelylarger delay period. The fundamental frequency is the frequency of thelowest periodic division which includes one such delay.

By effective frequency it is meant the frequency of a pure sinusoidalsignal which provides the same inductive heating effect as the currentpulse signal.

By high-frequency harmonics it is meant the harmonics at frequenciesabove (at a multiple of) the fundamental or root frequency.

A spectrum analyzer can be used to analyze a current pulse signal withhigh frequency harmonics. By way of comparison, FIG. 47 shows the waveform 700 of a single sine wave of amplitude A and frequency w, where thewave form is described by Asin(ωt+φ₀). FIG. 48 shows the frequencyspectrum 710 of this single sine wave 700, where all of the amplitude Ais carried by a single frequency ω. In contrast, FIG. 49 shows anexample of a current pulse signal with high-frequency harmonics 720(also referred to as a chopped wave). FIG. 50 shows the spectrum 730 ofthe chopped wave 720, which is a sum of cosine waves, starting with aroot frequency ω of amplitude α, and the high frequency harmonics abovethe root frequency of 2ω and amplitude α₂, 3ω and amplitude α₃, 4ω andamplitude α₄, etc. The amplitudes are generally decreasing as thefrequency increases. Preferably, the amplitudes stay high as thefrequency increases.

In a heater circuit, the two things that generally dictate the amount ofpower (heat) generated are the frequency and the current. The currenthas a much bigger effect than the frequency as seen by the equation:

$P = {I^{2}\sqrt{\omega}}$Thus, preferably, the current is kept high while increasing thefrequency.

The current pulse signal with high frequency harmonics is a wave withsteep edges and long pauses between the jumps in voltage. It may bereferred to as a chopped wave. The chopped wave can provide ten timesthe power of a sine wave of the same root frequency where the amplitudeof the high frequency harmonics is kept high.

In summary, the “root frequency” is the smallest time one can break awave into and still have it be periodic. The high frequency harmonicsare waves of frequency above the root frequency and together with theroot frequency “build” the desired wave. Generally, it is desirable isto generate large amplitudes within the harmonics so that the powerstays high. It can be desirable to use a root frequency of 50-60 Hzbecause it is readily available from the grid; the power supply can then“chop” the sinusoidal wave coming off the grid to generate the highfrequency harmonics that are desired.

A current pulse signal with high frequency harmonics has been describedas including both the fundamental (root) frequency, or first harmonic,and higher harmonics above the root frequency. The signal may thus beunderstood as being constructed from such components. Such constructionshould be understood to include, in the physical world, constructing apulse signal by starting with a root frequency signal (e.g., sinusoidal)and removing portions of waves to retain one or more harmoniccomponents. It would also include, for example, starting from arectangular pulse and changing the shape of the rectangular pulse.

In addition, the previous examples (FIGS. 3-6) show one pulse generatedin each half-period of the sinusoidal signal. However, the sinusoidalsignal could alternatively be “chopped” multiple times per half-period,generating multiple pulses per half-period. Also, instead of using abipolar switch, one can first rectify the sinusoidal signal with a diodebridge, before chopping the signal (one or multiple times eachhalf-period).

Select embodiments described herein utilize a cooling medium forreducing the temperature of the heated article, e.g., intermittently,while not heating the article. FIG. 51 shows schematically a heating andcooling apparatus 780 having a control circuit 781 to alternately and/orsimultaneously, as desired, supply a cooling medium from a coolantsupply and regulator 782, and supply a current pulse signal from a pulsegenerator 783, to an apparatus 784 which includes a heating element, acooling passage and an article to be heated and cooled.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope of theinvention being indicated by the following claims.

1. A method of controlled heating comprising: providing a heater coilinductively coupled to a ferromagnetic substrate; supplyingnon-sinusoidal current pulses having steeply varying portions providinghigh frequency harmonics in the heater coil for inducing an eddy currentto heat the ferromagnetic substrate; transmitting the heat from theferromagnetic substrate to an article to be heated; and intermittentlycooling the ferromagnetic substrate by reducing the signal supplied tothe heater coil to reduce the induced heating.
 2. The method of claim 1,wherein: the intermittent cooling includes one or more of: supplying acooling medium to cool the substrate, and drawing heat from thesubstrate.
 3. The method of claim 2, wherein: the cooling medium issupplied within the substrate.
 4. The method of claim 2, wherein: theheater coil is an electrically conductive tube having an electricallyinsulating cover in contact with the ferromagnetic substrate, and thecooling medium is supplied within a bore of the tube to cool theferromagnetic substrate.
 5. The method of claim 1, wherein: the pulsesignal generates primarily inductive heating power.
 6. The method ofclaim 2, wherein: the generated power includes less than 10% resistivepower.
 7. The method of claim 1, wherein: the generated power includesless than 5% resistive power.
 8. The method of claim 7, wherein: thegenerated heating power includes less than 1% resistive power.
 9. Themethod of claim 1, wherein: the ferromagnetic substrate has a layerthickness of no greater than about 3δ, where δ is the depth ofpenetration of the induced eddy current.
 10. The method of claim 9,wherein: the layer thickness of the substrate is about 3δ.
 11. Themethod of claim 1, including: providing a thermally insulating outerlayer; providing a thermally conductive inner layer for transmittingheat from the substrate to the article; and wherein the ferromagneticsubstrate is between the outer and inner layers.
 12. The method of claim1, including: providing a thermally insulating and ferromagnetic outerlayer; providing a thermally conductive inner layer for transmittingheat from the substrate to the article; and wherein the ferromagneticsubstrate is between the inner and outer layers.
 13. The method of claim1, wherein: a flow passage is provided in or adjacent to theferromagnetic substrate; and the heater coil and ferromagnetic substrateprovide controlled heating of a flowable material in the flow passage.14. The method of claim 13, wherein: the ferromagnetic substrate is partof a nozzle or melt channel.
 15. The method of claim 13, wherein: theferromagnetic substrate is part of a compression molding apparatus. 16.A method for temperature control of a flowable material in a passage,the method comprising: providing a ferromagnetic substrate with apassage for a flowable material and a heater coil inductively coupled tothe ferromagnetic substrate; applying non-sinusoidal current pulseshaving steeply varying portions providing high frequency harmonics inthe heater coil for inducing an eddy current to heat the ferromagneticarticle to effect a rate of flow of the material in the passage; andintermittently cooling the ferromagnetic substrate by one or more ofreducing the applied signal to the heater coil and drawing heat from thesubstrate to effect the flow of the material in the passage.
 17. Themethod of claim 16, wherein: the intermittent cooling includes supplyinga cooling medium to cool the ferromagnetic substrate.
 18. The method ofclaim 17 wherein: the heater coil has a thermally conductive cover incontact with the ferromagnetic substrate, and includes a cooling passagethrough which the cooling medium flows.